Take a moment and think about creating a multi-column grid in a Photoshop comp. Have your palms started to sweat? Yes, creating grids in Photoshop is a pain indeed. Some designers just estimate and drag guides arbitrarily onto the stage. Others draw vector shapes, duplicate them to represent columns, then stretch them to fit their design. The hardy few who don't say things like, "I'm a designer, not a mathematician," generally use a little math and logic to calculate their grid. If you were to boil that math down, it probably ends up looking something like this:

I was sitting at my desk one day doing this exact equation when I thought, "Man, this looks just like code. I wish someone would make a plugin that would do this for me." Several months and many grids later, it occurred to me that I could probably build the plugin myself.